It is known that electric power systems which have large inherent system reactance and/or supply highly reactive loads are characterized by poor voltage regulation, i.e., substantial change in the magnitude of load voltage as load current increases. In a typical inductive circuit voltage magnitude and power factor both decrease as load current increases. To improve voltage regulation power transformers are commonly provided with tap changers to counteract the tendency of voltage magnitude to change with change in load current. Since most system loads are inductive it is known also to counteract the inductive current components of system load or of particular major loads by connecting compensating capacitance in series with or in shunt across power line conductors. Fixed capacitors may be used where load is reasonably predictable.
With certain variable and erratic major loads, such as electric arc and induction furnaces, controllable shunt capacitance has been provided by connecting rotating synchronous condensors or static capacitors directly across the load terminals in parallel with the load. The amount of capacitance must be varied as load current changes, for fixed capacitance would have the effect on no load of increasing load terminal voltage above the applied system voltage. The response time of rotating equipment is too slow however to prevent undesirable lamp flicker on the line as a result of load induced voltage variation. Similarly, mechanical switching means used for controlling static shunt capacitors does not respond sufficiently rapidly to prevent flicker. While it is known that solid state power switches may be made to respond within less than half a cycle of the power frequency, their use directly in circuit with compensating shunt capacitors is not entirely satisfactory; the leading capacitive current leaves residual charge in the capacitors and as a consequence troublesome transient voltages or harmonic frequencies are generated.
Several arrangements have recently been proposed for varying the net reactive current effect of fixed shunt compensating capacitors by connecting compensating inductors in parallel with the capacitors and varying the amount of reactive current traversing the inductors. This may be done by varying the magnitude of the shunt inductance across each line, as in patent 3,551,799-Keppleman, or by varying the amount of reactive current traversing shunt inductors of fixed magnitude, as in "Electric Technology U.S.S.R.", Vol. 1 Oct. 1969 pages 46-62 (Pergamon Press, October, 1969). Such an arrangement is shown in U.S. Pat. No. 3,936.727 - Kelley and Lezan.
I have discovered that when a three phase or other multiphase inductive compensating reactor including phase controlled alternating current static switches is connected to a power line in the manner illustrated in the foregoing Kelley & Lezan application a number of undesirable harmonic frequency currents are generated by phase control of current in the inductive compensating reactors. In a three phase reactor with bilaterally conductive static switches providing six current pulses per cycle the predominant harmonic frequencies are the 5th, 7th, 11th, 13th, 17th, 19th, 23rd and 25th harmonics of power system frequency. These harmonic frequencies F.sub.h may be expressed by the relation F.sub.h = 6K .+-. 1, where K is a positive integer. In a three phase inductive compensator of the static phase controlled type these characteristics exist whether the compensating inductors are delta connected or wye connected.